High energy densities with low weight have been achieved in lithium secondary batteries (typically, lithium ion batteries) in recent years, and hence lithium secondary batteries are growing in importance as high-output power sources for vehicle mounting or power sources for power storage systems. In one typical configuration of this type of lithium secondary battery, the battery has electrodes (positive electrode and negative electrode) in which an electrode active material layer containing an electrode active material is supported on an electrode collector. The electrodes are typically formed by coating the surface of the collector with a paste for forming an electrode active material layer containing an electrode active material, a binder and so forth, drying and subsequent rolling to a predefined density (so-called coating method).
Graphite materials such as natural graphite, synthetic graphite or amorphous carbon from natural graphite or synthetic graphite are widely used as the negative electrode active material contained in such negative electrodes. The graphite material has a layered structure in which planes (referred to as graphene and corresponding to the (002) plane of a graphite crystal structure), made up of carbon six-membered rings, form a stack of a plurality of planes, such that lithium ions become intercalated (stored) between layers of the layered structure and de-intercalated (released) from between the layers, to elicit charge and discharge as a result. Various approaches have been conventionally proposed in order to enhance the performance of negative electrodes in which such graphite material is used as the negative electrode active material.
For instance, Patent document 1 discloses the feature of arranging the direction of the (002) planes of graphite particles in a direction perpendicular to a collector, with a view to enhancing the high-rate discharge characteristic and permeability of an electrolyte solution in a negative electrode, in a case where graphite particles are used as the negative electrode active material.
Patent documents 2 and 3 disclose the feature of producing a negative electrode for lithium secondary batteries by solidification molding of a graphite powder, using a binder material, through removal of a solvent in a state where the (002) planes of graphite particles contained in the graphite powder are oriented in the same direction, in a magnetic field, with a view to increasing discharge capacity and enhancing a cycle characteristic during high-rate charge and discharge.
Patent document 4 discloses the feature of using graphite particles having an average particle size of 10 to 25 m and a specific surface area of 1.0 to 5.0 m2/g, as a negative electrode active material that is oriented in a magnetic field, with a view to enhancing a fast charging characteristic and a high-rate discharge characteristic.